115
In northern Japan, many species of the genus Zostera other than Z. marina (e.g.,
Z. caulescens, Z. caespitosa, and Z. asiatica) are often dominant (Nakaoka and Aioi
2001). In general, these species form their beds in deeper zones than Z. marina, and
there has been little information available on biomass and production for these species. For Z. caulescens in Otsuchi Bay off the Sanriku coast, annual production was
estimated to be 0.47 kg dry weight/m
2
/year (Nakaoka et al. 2003). On the other
hand, in Akkeshi Bay in eastern Hokkaido, Z. asiatica had a mean production of
2.03 kg dry weight/m
2
/year, which exceeded the value (1.35 kg dry weight/m
2
/year)
of the sympatric Z. marina (Watanabe et al. 2005). The latter case points out the
possibility of over- or underestimation of the production of eelgrass beds composed
of multiple species when the values are extrapolated from production data of only
Z. marina.
Until now, data on the actual production of seagrass in Japan have been still few.
To clarify the mechanism of the spatiotemporal variation of production of eelgrass
beds, continuous surveys of beds exhibiting various characteristics are necessary, as
is monitoring of the fluctuation of environmental parameters related to production.
4.3.2 Sargassum Bed (Garamo-ba)
4.3.2.1 Measurement of Production by Productive Structure Analysis
Thalli of Sargassum adheres to hard substrata such as rocks or boulders by holdfasts
as it grows. A short axis (a stem or stipe) arises from the holdfast, and several main
branches are formed at the top of the axis and elongate. Thalli develop from 1 m to
several meters in length, but the length differs among species. Lateral branches and
leaves are formed on the main branches, and buoyant vesicles transformed from
leaves are formed to allow the thalli to stand upright (Fig. 4.2). The presence of the
complex morphological features of the constituent thalli means that Sargassum
beds present an extremely complex three-dimensional structure that offers an
important habitat for many organisms.
Sargassum exhibits extremely clear seasonal growth, and after the growth season
in which the main branches rapidly elongate, reproductive organs (receptacles) are
formed on the branches in the maturation season. Generally, the maturation season
of many species in Japan is in spring, and the biomass also reaches the yearly maximum during this season. Most Sargassum species (e.g., S. macrocarpum, S. patens,
and S. siliquastrum) are perennial, but a few are annual (e.g., S. horneri). Whole
thalli of S. horneri die back after maturation, so beds of S. horneri temporarily disappear in summer. On the other hand, although the old main branches of perennial
species also wither after maturation, newly formed main branches sprout out from
the persistent perennial systems of holdfasts and stipes. Therefore, beds of perennial
Sargassum are maintained throughout the year.
Sargassum beds often develop to be large in scale and offer important fishing
grounds in Japan. Therefore, there have been numerous ecological investigations of
4 Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future…
In northern Japan, many species of the genus Zostera other than Z. marina (e.g.,
Z. caulescens, Z. caespitosa, and Z. asiatica) are often dominant (Nakaoka and Aioi
2001). In general, these species form their beds in deeper zones than Z. marina, and
there has been little information available on biomass and production for these species. For Z. caulescens in Otsuchi Bay off the Sanriku coast, annual production was
estimated to be 0.47 kg dry weight/m
2
/year (Nakaoka et al. 2003). On the other
hand, in Akkeshi Bay in eastern Hokkaido, Z. asiatica had a mean production of
2.03 kg dry weight/m
2
/year, which exceeded the value (1.35 kg dry weight/m
2
/year)
of the sympatric Z. marina (Watanabe et al. 2005). The latter case points out the
possibility of over- or underestimation of the production of eelgrass beds composed
of multiple species when the values are extrapolated from production data of only
Z. marina.
Until now, data on the actual production of seagrass in Japan have been still few.
To clarify the mechanism of the spatiotemporal variation of production of eelgrass
beds, continuous surveys of beds exhibiting various characteristics are necessary, as
is monitoring of the fluctuation of environmental parameters related to production.
4.3.2 Sargassum Bed (Garamo-ba)
4.3.2.1 Measurement of Production by Productive Structure Analysis
Thalli of Sargassum adheres to hard substrata such as rocks or boulders by holdfasts
as it grows. A short axis (a stem or stipe) arises from the holdfast, and several main
branches are formed at the top of the axis and elongate. Thalli develop from 1 m to
several meters in length, but the length differs among species. Lateral branches and
leaves are formed on the main branches, and buoyant vesicles transformed from
leaves are formed to allow the thalli to stand upright (Fig. 4.2). The presence of the
complex morphological features of the constituent thalli means that Sargassum
beds present an extremely complex three-dimensional structure that offers an
important habitat for many organisms.
Sargassum exhibits extremely clear seasonal growth, and after the growth season
in which the main branches rapidly elongate, reproductive organs (receptacles) are
formed on the branches in the maturation season. Generally, the maturation season
of many species in Japan is in spring, and the biomass also reaches the yearly maximum during this season. Most Sargassum species (e.g., S. macrocarpum, S. patens,
and S. siliquastrum) are perennial, but a few are annual (e.g., S. horneri). Whole
thalli of S. horneri die back after maturation, so beds of S. horneri temporarily disappear in summer. On the other hand, although the old main branches of perennial
species also wither after maturation, newly formed main branches sprout out from
the persistent perennial systems of holdfasts and stipes. Therefore, beds of perennial
Sargassum are maintained throughout the year.
Sargassum beds often develop to be large in scale and offer important fishing
grounds in Japan. Therefore, there have been numerous ecological investigations of
4 Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future…